Referring to FIG. 1, in a large general purpose computer system, the large number of electronic components 10 which make up the computer system generally requires that the components be arranged on more than one printed circuit board 12. Typically these printed circuit boards 12 are arranged in parallel within a card cage 14. Tracks 17 within the card cage 14 guide the printed circuit board 12 during the insertion of the board into the card cage 14.
One end of the card cage 14 is closed by a motherboard 16 upon which connectors 18 are located. The connectors 18 supply electrical power and ground connections to each of the printed circuit boards 12 and permit each of the printed circuit boards to communicate with other printed circuit boards 12. Each printed circuit board 12 has an edge connector 20 containing a set of contact pins 21 located on the edge of the printed circuit board 12 which bring electrical power, ground and signals to the electronic components 10 on the board 12. Each edge connector 20 on each printed circuit board 12 mates with one of the connectors 18 on the motherboard 16. Generally the motherboard 16 is located opposite the opening in the card cage 14 in through which the circuit boards 12 are inserted. The pins in the motherboard connector 18 are oriented in the direction of insertion of the printed circuit board 12. Each of the individual pins 21 in the edge connector 20 on the printed circuit board 12 mates with a corresponding pin within the motherboard connector 18. The pins within the motherboard connector 18 are constructed to tightly grip the pins 21 on the edge connector 20 and a good deal of force is required to mate the pins 21 on the edge connector 20 with the pins in the motherboard connector 18. This force is directed down onto the circuit board during insertion and causes undesirable bowing and related stress to the electrical leads and components that increases the risk of failure.
The force used in inserting the edge connector pins 21 into the motherboard connector 18 also causes the pins of the connector 18 to rub against the pins 21 of the edge connector 20 and thereby remove or wipe any oxide from the pins 21 and the pins of the motherboard connector 18. In so doing, this action provides better electrical contact between the pins 21 of the edge connector 20 and the pins in the motherboard connector 18. The use of such motherboard connectors and circuit board edge connectors permits the circuit boards 12 to be removed and replaced easily.
In addition, each circuit board 12 may have an insertion/removal lever 24,25 located at each outer corner of the board 12. One end of the top lever 24 engages the upper surface of the cage 14, while one end of the bottom lever 25 engage the lower surface of the cage 14. These levers 24, 25 aid in forcing the pins 21 of the edge connector 20 into the motherboard connector 18 during circuit board 12 insertion and in pulling the pins 21 of the edge connector 20 from the motherboard connector 18 during circuit board 12 removal. The top and the bottom surfaces of the card cage 14 typically also have slots 22 through which air flows vertically (arrows A and A') to cool the components 10 on the boards 12.
While the arrangement just described is used to mate a board 12 to a motherboard 16 whose connector pins 18 are oriented in the direction of insertion, a different arrangement permits a circuit board 12 to be inserted into the card cage 14 in a direction perpendicular to the orientation of the pins of the connector 18. Generally such an arrangement is used to permit air to flow horizontally along the length of the board rather than vertically along the height of the board.
Such a horizontal air flow is important when multiple card cages are stacked vertically. With a vertical arrangement of card cages, hot air, from the card cages below, is forced through the card cages above, resulting in less cooling to the circuit boards in the upper card cages. Vertical tiers of horizontal motherboards allow fresh, cool air flow to be directed across each stack of cards in their cages.
This form of circuit board insertion is typically accomplished using a zero-insertion force (ZIF) connector. A ZIF connector is constructed to be opened prior to circuit board insertion so as to increase the space between the pins of the motherboard connector. This permits the edge connectors 20 of the circuit board to be slid between the pins of the motherboard connectors 18. Once the circuit board 12 has been slid into the space between the motherboard connector pins, the connector is closed bringing the pins of the motherboard connector into contact with the pins 21 of the edge connector 20. However, since the pins 21 of the edge connector 20 do not rub against the pins of the motherboard connector during insertion, any oxide which has been formed on the pins remains and thereby decreases the reliability of the connection.